The present invention relates to a process for roughening and chromium-plating, in the same electrolyte, steel and steel-based plates for lithographic applications, e.g., in the production of offset printing plate supports.
Offset printing plates which, for simplicity, are hereinafter referred to as printing plates, are generally comprised of a support to which at least one radiation-sensitive reproduction layer is applied. The reproduction layer is applied to the support either by the user, in the case of plates which have not been precoated, or by the industrial manufacturer in the case of precoated (presensitized) plates.
The printing plate supports predominantly used comprise metallic materials, principally aluminum and its alloys. However, support plates of normal carbon steel or steel alloys, for example, chrome-nickel steels, manganese steels and the like, are also used.
In order to obtain certain necessary printing plate properties, such as an adhesive capacity for the layer, differentiation of hydrophilic and hydrophobic areas with defined behavior, corrosion properties and surface hardness, which is important for the length of the printing run, the printing plate is, in general, subjected to a pretreatment. This pretreatment includes, for example, a modification by mechanical, chemical or electrochemical roughening, which is also referred to as graining or etching, chemical or electrochemical surface oxidation, treatment with agents which confer hydrophilic properties, or heat hardening.
A combination of the above types of modification is frequently used in the modern, largely continuously operating high-speed units of the manufacturers of uncoated or precoated printing plates.
When aluminum or aluminum alloys are used, the modification comprises, in most cases, a combination of mechanical and/or electrochemical roughening and an anodic oxidation, followed, if appropriate, by a stage in which the plate is rendered hydrophilic.
Aluminum-based support plates are widely used and have proven largely satisfactory, even though, due to the material, they have a lower mechanical strength and wear resistance than steel plates. However, the aluminum-based carrier plates are not amenable to advantageous magnetic fixing to the printing cylinders. The desirable property of magnetic fixing is of particular interest for high-speed rotary presses.
In order to eliminate, in particular, this disadvantage, processes have been developed for the production of sandwich plates based on aluminum or steel, particular attention being paid to plates with chromium layers, because of their surface hardness. For example, German Offenlegungsschrift No. 2,544,295 discloses sandwich plates composed of a base support of aluminum or steel, on which printing and non-printing areas comprising two different metals are present. The printing areas mainly comprise copper, and the non-printing areas comprise chromium.
European patent application No. 20,021 discloses a process for the production of chromium-plated metal plates which are comprised, inter alia, of steel and which are used for lithographic purposes. The disclosed printing plate is produced by the following steps:
Initial cleaning PA0 Rinsing PA0 Roughening PA0 Rinsing PA0 Chromium-plating PA0 Rinsing PA0 Drying PA0 Coating. PA0 Cleaning PA0 Initial chromium-plating with direct current PA0 Rinsing PA0 Main Chromium-plating with direct current PA0 Rinsing PA0 Post treatment of the surface.
The roughening is carried out in a solution of bifluorides, and the cathodic chromium-plating is carried out in a bath of chromium oxide and sulfuric acid. The publication teaches that the conditions for the rinsing steps are critical, since otherwise uniform plates are not obtained.
European patent application No. 97,502 discloses the chromium-plating of steel plates as base supports for lithographic purposes, comprising the following steps:
The initial chromium-plating is carried out in a bath of chromic acid and nitric acid, and the main chromium-plating is carried out in a bath of chromic acid and sulfuric acid, the current density and the temperature being higher in the second bath. The posttreatment of the surface is carried out using a solution of a water-soluble polymer, such as gum arabic, and a water-soluble salt, such as zinc acetate. Here again, the publication emphasizes that the rinsing steps are of special importance.
A similar multi-stage treatment for the production of chromium-plated steel plates is disclosed by European patent application No. 97,503. The essential difference between the aforesaid multi-stage treatment process and that disclosed by European patent application No. 97,502 concerns the individual bath compositions. The above-mentioned processes have the disadvantages that both are multi-stage processes and, in particular, that care must also be taken during the interposed rinsing steps. Moreover, the baths used in the individual stages must be very precisely matched, since otherwise the end products do not meet the demands of industry. Viewed as a whole, the known processes are expensive to implement and, as a result, lead to increased costs in the production of the plates.
Disposal of the spent baths, each of very different composition, and/or the rinsing water also engenders effluent problems. Moreover, the plates have peak-to-valley heights which are not optimal for anchoring the resist layer.